Study on emission yields of negative- and positive-ion induced secondary electron from thin SiO2 film

The emission yield of secondary electrons from thin SiO2 films due to negative- and positive-ion bombardments was studied experimentally for the purpose of the investigation of the charging phenomenon of insulators during the negative-ion implantation. We used 120- and 360-nm-thick SiO2 films on Si and low ion currents for obtaining the true yield for the insulator without charging effect. The yields (the current ratio of emitted electrons to incident ions) were measured as a function of the C- current density. Although the measured yield for each ion energy showed the value of unity at a high ion current density, it was saturated to the same value irrespective of the film thickness with a decrease in ion current density. It is considered that the charging effect was reduced by compensation due to the leakage current and that the true yield was obtained at a sufficiently low ion current density. As results from C-(C-2(-)) and C+ ion bombardments, the true yield for negative ions was found to be larger (approximately by one unit! than that for positive ions due to the detachment of a loosely attached electron from the negative ion. The emission mechanism of secondary electrons for negative ions is considered to consist of kinetic emission related to ion momentum and emission related to electron detachment of negative ions.